This invention relates to a method for growing single crystals of gallium arsenide and, more particularly, to a method for growing semi-insulating single crystal gallium arsenide with improved physical characteristics using boron oxide encapsulant.
Gallium arsenide (GaAs) single crystals can be grown by any of a number of methods. Many of these methods use a liquid encapsulant, usually boron oxide (B.sub.2 O.sub.3), to improve the quality of the single crystal GaAs. When grown under a layer of molten B.sub.2 O.sub.3, the evaporation of the more volatile component of GaAs is better controlled, stoichiometry of the single crystal is improved and the impurity content of the GaAs is lowered.
It is generally acknowledged in the industry, as witnessed by the published literature, that it is essential that the B.sub.2 O.sub.3 must be substantially anhydrous in order to provide the above recited improvements. For instance, it has been stated that "it is essential to remove traces of water from the boron oxide" (J. Phys. and Chem. Solidsm 26, 4, 782 (1965). Similar statements have been made during the 1966 Symposium on GaAs (Chemical Abstracts 69, 46579 y), in the J. Crystal Growth, 3, 4, 281 (1968), in the Mat. Resources Bull., 6, 1297 (1971) and in the J. Electr. Matl., 4, 2, 389 (1975). The water content of B.sub.2 O.sub.3 is generally lowered by subjecting the B.sub.2 O.sub.3 to vacuum baking for a period of time at temperatures as high as 1100.degree. C. In spite of the special treatment of B.sub.2 O.sub.3 to reduce its water content, the water content of B.sub.2 O.sub.3 used in crystal growth is not entirely eliminated and may range from substantially anhydrous, i.e. less than 100 ppm, to as high as 1000 ppm. This residual water content of B.sub.2 O.sub.3 has a distinct effect on the physical properties of GaAs.
It is desirable that semi-insulating GaAs crystal material has physical properties which do not cause instability. It is particularly desirable that such GaAs has a resistivity which does not decrease when the material, in the form of wafers, is subjected to a heat treatment to obtain suitable characteristics for electronic devices on the GaAs. Some GaAs material shows a decrease in resistivity upon heat treatment, while other GaAs material shows an increase in resistivity or possesses a constant resistivity, the latter material being the preferred material.